You see a person you're attracted to, walk up to them and say "hi, I'm X, and what's your name?", and then you watch for the signals. If the person squirms and shows signs of being uneasy, you might proceed in two ways:
1) try to find a common topic of conversation - for instance, you notice the person wears a Green Day patch, and you swallow your disgust at the band (I should hope so) and comment on the excellent way the patch is incorporated into the overall aesthetics of their clothing
2) the GTFO route - just mumble something and get the fuck out of that awkward situation
Repeat ad nauseam until you stumble across a person showing good signs, and then:
1) the subtle and mysterious route - produce a witty comment on just how you'd like to continue the interaction in a way or another, but DO NOT ask for a number or give yours (this, of course, depends on the information you were provided with, and if you have an idea of where can you see the person, for instance, regular places of visit like cafes, then and only then this route is an option), and an appreciative comment, alongside the expression of a wish to see the person again, is obligatory when you say that you gotta go now. I should also note that this route also depends on your physical appearance.
2) gimme yer number route - but be careful not to be pushy, you can use language to your own advantage and say something like "I was wondering whether you'd like to go for a cup of coffee or something, but then I'd be needing your number lol".
There is more advice, and you'll get it soon.
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